Dopamine is a crucial neurotransmitter in the nervous system that regulates activities including movement, olfaction, and mood. Dopamine is derived from tyrosine via a series of enzyme-catalyzed reactions. The biosynthesized dopamine is transported to vesicles via the vesicular monoamine transporter (VMAT) for storage. When a nerve impulse is conducted to the synapse, the vesicles that docked at the active zone fuse with the plasma membrane and release dopamine to the synaptic cleft. The released dopamine binds to the dopamine receptors on the membrane of the postsynaptic neuron and transmits the nerve impulse to the downstream cell. Some released dopamine will be recycled back into the cells via dopamine transporters (DAT) that localized on the membrane of the presynaptic neuron or the nearby glial cells for metabolism or reutilization. Clinical studies and experimental animal models showed that alteration of dopamine metabolism could dramatically affect physiological conditions of the subject. For example, substances abuse such as administrating amphetamine could strongly affect cognition due to the alteration of dopamine concentration in the synaptic cleft. Importantly, neurological disorders, including Parkinson's disease, depression, and other mental illness have been linked to dopamine system. Using Parkinson's disease as an example, the hallmark pathology of the patients is the neuronal loss in the substantia nigra, where most neurons use dopamine to send message to the striatum for controlling motor functions. Thus, the loss of dopaminergic neurons could significantly reduce the amount of dopamine released to the synapse and subsequently lead to parkinsonism. Therefore, it is critical to understand dopamine metabolism and function in the brain for delineating dopamine-related disorders and physiology. Dopamine is not stable under physiological condition. Indeed, dopamine measurement in vivo remains a bottleneck for neuroscientist. Currently, the most common method to analysis dopamine is using high performance liquid chromatography (HPLC) to measure the biological sample in the non-physiological condition. This method is unable to distinguish dopamine from individual neuron, or to determine whether the readout is the secreted dopamine or dopamine inside the cell. Recently, the development of multi-walled carbon nanotubes (MWCNT) takes the advantage of that each substance has specific oxidation-reduction potential after being oxidized by the carbon nanotube, thus the possible content of the tested substance can be converted by the specific potential changes of the potential. However, this method requires the injection of carbon nanotubes into the tested subject, which is invasive and only suitable for a single point measurement. Moreover, the oxidation-reduction potential of many substances in actual measurements produces overlapped readings; for instance, the oxidation-reduction potential of vitamin C overlaps with dopamine, which may hinder the specificity of this measurement since vitamin C often coexists with dopamine in vivo.